1. Field of the Invention
The present invention relates to an aggregate blasting method for the excavation of a working face having one free surface in tunnel and particularly to a method of forming a second free surface comprised of these steps: drilling a number of slant holes around the central zone of a working face and a number of parallel cut holes with an area surrounded by the holes in a working face; loading an electric detonator and an explosive material in the holes and the parallel cut holes; and sequentially blasting the holes and center cut holes, middle cut holes and outer cut holes out of the parallel cut holes in order to form a cubical space, whereby a second free surface is easily formed.
2. Description of the Prior Art
Generally, with a blasting method for excavating the working face having one free surface, a method for drilling cut holes in order to obtain a second free surface is divided into an angle cut method and a parallel cut method according to a drilling method of cut holes.
The V-cut method in angle drilling and the burn cut method in parallel drilling are now generally used in a tunnel blasting.
The angle cut method is applied to the short hole blasting of soft rock and the burn cut method is applied to the long hole blasting of hard rock.
However, the burn cut method as shown in FIGS. 1A-1B has many problems as follows:
1) In order to increase the blasting efficiency based on a theory of blasting, the projective area of a pilot hole against a free surface must be large. But, the projective area of the parallel hole is small as shown in FIG. 5B, and the blasting efficiency is low because a blasting pressure acts on 40.degree..about.60.degree. against the working face 9 as shown in FIG. 17B. PA1 2) The relief hole 1 in FIG. 1A-1B is difficult to drill and the drilling must be done so that the spacing of relief holes 1 may be 5 cm to 7 cm. Accordingly, a high level of drilling technology and expensive drilling machines are required. Also, the interpenetration of relief holes may frequently occur. PA1 3) Because relief hole 1 has a large diameter, its drillng time is long. PA1 4) A rock is difficult to reinforce because rock damaged zone 6 is large, and a scaling time of fragmented rocks is long. PA1 5) A ventilation pipe, an electric panel, and a drill's water pipe are damaged because the fragmented rocks badly scatter. PA1 6) A number of the pilot holes must be formed because the projective area of the pilot hole is small. Also, since the explosive material must be tightly loaded, a large quantity of explosive material is required. PA1 7) The working time per excavation cycle is long. The cost of reinforcement and excavation is high because the excavation efficiency is low. PA1 1) The advance of the V hole is shorter than that of the parallel hole. So, the blasting efficiency is the lowest in the case of a long hole. PA1 2) A spacing of V holes on the hole bottom section must be about 100 mm. If the spacing of slant holes on the hole bottom section is more 100 mm, the blasting efficiency decreases. If the spacing on the hole bottom section is less 100 mm, the V holes might be interperforated. PA1 3) Because the holes are drilled with a dip, the hole bottom section and the drilling depth are not uniform. Thus, the rock damaged zone is large and the blasting pressure is large. Also, the possibility of an accident because of the fragmented rock is increased, and it takes long time to scale the fragmented rock. PA1 4) A number of the cut spreader holes 12 (see FIG. 2B) must be drilled to tightly load the explosive material in the center of the working face. PA1 5) The cost of reinforcement and excavation is high because the excavation efficiency is low.
Also, the angle cut method as shown in FIG. 2A-2B has many of the following problems:
Consequently, so far, the previous angle cut technique mainly depends on the procedure of increasing a projective area of the V hole in order to increase a blasting efficiency by the formation of a second free surface. However, the spacing of the V holes on the hole bottom section must be about 100 mm and the loading density of the explosive material must be large.
To obtain two free surfaces, the blasting method by the parallel cut must include the steps of drilling the relief hole 1, the hole diameter of which is .phi.102.about.120 mm, and the cut holes 2, the spacing of which is 5.about.7 cm, loading the explosive material in the cut holes 2, and blasting the cut holes 2. Alternatively, to form a second free surface, an unloaded hole having a large diameter is drilled by a tunnel boring machine.
Accordingly, the previous conventional techniques require a high level of technology and machinery.